(1) Field of the Invention
The present invention relates to a method of producing grain-oriented silicon steel sheets, and more particularly, to a method of producing grain-oriented silicon steel sheets, which can be easily separated from an annealing separator after final annealing and have substantially no glass film and have excellent punchability and magnetic properties.
(2) Description of the Prior Art
Grain-oriented silicon steel sheets have hitherto been produced by a series of steps, wherein a silicon steel containing not more than 4.0% of silicon is hot rolled, the hot rolled sheet is annealed and then subjected to one cold rolling or two cold rollings to produce a cold rolled sheet having a final gauge, the cold rolled sheet is subjected to a primary recrystallization and concurrently to a decarburization annealing to form subscale consisting mainly of SiO.sub.2 on the steel sheet surface, an annealing separator consisting mainly of MgO is applied to the steel sheet, the above treated steel sheet is subjected to a final annealing to develop secondary recrystallized grains having (110) (001) orientation and concurrently to remove harmful impurities and to form forsterite glass film having a thickness of 2-5 .mu.m.
In general, a phosphate series insulating coating is formed on the above described forsterite glass film to produce a final steel sheet product. However, since the forsterite film has a very high hardness, a metal die is seriously worn during the punching of steel sheets having the forsterite film. Therefore, when rigid glass film is not formed during the final annealing, the wear of metal die during the punching can be prevented, and the punchability of the metal die can be improved. The punchability of a metal die is represented by the number of punched sheets without wearing of the die.
Further, when the glass film of grain-oriented silicon steel sheet is removed and the steel sheet is mirror-finished as commonly known, the magnetic induction of the steel sheet is remarkably increased, and the iron loss thereof is noticeably improved at the same time. Treating methods for steel sheet, which do not form glass films on the steel sheet during the final annealing, are demanded so that the mirror finishing of the finally annealed steel sheet can be easily carried out.
In any of methods proposed as a treating method, which does not form glass film with the use of an annealing separator consisting mainly of MgO, glass film is locally formed and a strong pickling must be carried out in order to remove completely the remaining glass film.
When an annealing separator consisting mainly of Al.sub.2 O.sub.3 is used, since Al.sub.2 O.sub.3 is inactive, glass film is not newly formed, but subscale formed during the decarburization annealing is aggromelated into coarse particles, and the coarse particles remain near the surface of the finally annealed steel sheet, or the sticking of Al.sub.2 O.sub.3 on the surface occurs. Therefore, a strong pickling must be carried in order to remove the coarse particles or the sticked Al.sub.2 O.sub.3. Such strong pickling not only consumes acid, but also solves excessively steel sheet. One of methods for extinguishing subscale remaining in a steel sheet is to carry out decarburization annealing under an atmosphere having a low oxidizing ability, whereby the thickness of the resulting subscale is made as small as possible and oxide film is formed only on the surface of the steel sheet. However, in order to form such thin oxide film, an atmosphere having an extremely low oxidizing ability must be used, and as the result the decarburization can not be sufficiently carried out and the magnetic properties of the resulting grain-oriented silicon steel sheet is poor.
The inventors have already disclosed in Japanese Patent Laid Open Application No. 22,113/78 a method for producing grain-oriented silicon steel sheets having metallic luster and further having excellent punchability and magnetic properties, and an annealing separator, which is effectively used in this method and can be easily peel off from the finally annealed steel sheet.
The inventors have made further investigations for improving the method of the above described Japanese Patent Laid Open Application No. 22,113/78, and accomplished the present invention.